When you are using the navigation feature, there seems to be two things you need to do to keep that battery charged.
1. Keep the Droid plugged in using an appropriate charger; for me the Verizon Charger works well, as does the RocketFish (Best Buy) flush mount USB; the Belkin I tried did not supply enough amperage to keep it charged for me.
2. Keep the Droid cool. When it gets too hot, the charging circuit shuts down. Some folks mount the Droid on A/C vent; others just keep it out of the sun. I have a Honda Element and I mount my Droid on the dash just above the 2 center AC vents. I separated the top fin on both vents so the airflow from these two fins is always directed up and hits the Droid. Since doing this, I have not encountered any time when the charging circuit shut off, even on a 5-hour trip between Richmond, VA and Myrtle Beach in the bright sun with the outside temps in the mid 90s.
Just my experience, yours may differ.
As stated above, if overheating, your battery will not charge. The only thing you can do in such cases is allow the device to cool off.
I recently discovered a great workaround for this problem. I mount my phone in a window dock positioned with the phone on the far left end of the windshield. This positions the phone in the full stream of the vent that points towards the driver's side window.
Among the many oversights in the Droid O/S, charging behavior has to be one of the worst. It's fine for the phone to stop charging to prevent overheating. It's NOT fine for the phone to do so without warning its owner.
I use the GPS all the time and have adopted two work arounds for this issue.
1. Using a small piece of plastic (2" x 1") I formed a deflector that slides into the AC vent and directs a small amount of air to wash over the windshield mount of the droid. It doesn't take much air to cool the device while in GPS but on a hot day with no air flow that area and the phone get quite warm in direct sunlight...maybe the windshield mount shouldn't be black!
2. Using a 6" x 6" piece of a notbook paper placed over the back of the windshield mount keeps the phone and the black mount out of the sun and in the shade. The paper has no impact on the GPS reception and all seems to work good unless the car is sitting in direct sunlight for a long time...use method #1 above.
Treo600, Treo 700W(X), Omnia 910, Moto Droid1, Moto Droid2(G), Moto Droid4
easy fix: car vent mount.
Radio shack sells a Generic mount that works really well. You can move the supports around such that all buttons can be used.
The back of the phone stays cool to the touch with this setup.
Bracketron™ PHV-202-BL GPS Mobile Grip-It Vent Mount - RadioShack.com
I've actually noticed this as well on my Droid. My battery seems to drain when I'm using navigation with the phone plugged into the stock Verizon phone charger.
this is what I use. $11.99 @ pepboys. Vent mount, pinch style, rotating....
it slips out of the bottom mount thing that can be mounted to a dash.... has the standard vent mount